Semiconductor devices having partially-thinned structures as typified by MEMS (microelectromechanical system) include MEMS pressure sensors and MEMS accelerometers. Generally, a plurality of such sensors is concurrently formed in a semiconductor wafer process so as to acquire a diaphragm structure or a beam structure, and is subsequently individually separated. As for the separation, the most common method involves rotating a circular dicing saw to which diamond or c-BN particles are secured at high speed to perform a fracturing process. The process is performed while running water for removing fractured waste and suppressing frictional heat. However, since diaphragm structures and beam structures are structurally fragile, there is a risk that the pressure generated by the water may destroy the structures.
Recently, processing using a laser beam has received attention as a separation method not requiring water. Japanese Patent No. 3408805 discloses a method involving using a laser beam to form a modified region in a semiconductor substrate through multiple photon absorption and performing separation at cleavages originating at the modified region. Multiple photon absorption is a phenomenon in which absorption occurs in a material when light intensity is significantly increased even in a case where light energy is lower than an absorption band gap of the material or, in other words, in a case of entering an optically transmissive state.
For example, as shown in FIGS. 19A and 19B, a focal point of a laser beam 108 is set to the inside of a semiconductor substrate 101 at a portion of a separating line 104 that individually separates a plurality of semiconductor elements 102 formed on the semiconductor substrate 101 to cause multiple photon absorption in a thickness direction. Subsequently, by scanning the laser beam 108 along the separating line 104 so as to continuously or intermittently cause such multiple photon absorption, a modified region 109 along the separating line 104 is formed inside the substrate and a crack 110 originating at the modified region 109 is created. With this arrangement, by simultaneously applying external force on both sides of the separating line 104, the semiconductor substrate 101 can be easily split even with a relatively small external force. In a case where the semiconductor substrate 101 is thin, splitting occurs naturally at the crack 110 even when no external force is applied.
However, with this method, if the semiconductor substrate 101 is thick, splitting cannot be achieved depending on the modified region 109 produced by a single scan. Therefore, a plurality of scans must be performed so as to approximately serially align the modified regions 109 of the respective scans in a thickness direction, resulting in an increase in process takt.
Another method is to reduce the thickness of a processed portion by forming, in advance, a groove on a separating line through anisotropic etching or the like. For example, in a method disclosed in Japanese Patent Laid-Open No. 2001-127008, anisotropic etching is performed after forming an etching protective film on a semiconductor substrate on a (100)-oriented surface so as to expose longitudinal and lateral separating line portions. As a result, etching is stopped at a (111)-oriented surface and a V-groove having an inclination angle of 54.7 degrees is formed. By applying external force on the semiconductor substrate so as to expand the V-groove, the semiconductor substrate can be separated along the V-groove, i.e., along the separating line.
However, with this method, since the erosion due to anisotropic etching of portions at which the longitudinal and lateral V-grooves intersect differs from the erosion of other portions, excessive etching may prevent etching from stopping at the (111)-oriented surface and etching may proceed to a (211)-oriented surface. For example, when concurrently forming V-grooves in a process of forming a diaphragm structure requiring deeper etching than that required for V-grooves, intersecting portions of the V-grooves end up being excessively etched, sometimes to the extent of penetrating the semiconductor substrate. Consequently, the strength of the semiconductor substrate deteriorates significantly, causing the semiconductor substrate to break during handling.
Japanese Patent Laid-Open No. 2004-186340 discloses the formation of continuous-line first grooves and broken-line second grooves as scribe grooves on separating lines on a substrate. Japanese Utility Model Laid-Open No. H04-109537 discloses the formation of continuous cut grooves and discontinuous cut grooves on separating lines of a substrate on which a semiconductor device having a diaphragm is formed. However, in the case as described above where only grooves on separating lines in either the longitudinal direction or the lateral direction are formed as continuous grooves, since the continuous grooves are not uniformly disposed with respect to the four sides of individual semiconductor devices to be separated, stress tends to concentrate on sides on which the continuous grooves are formed and damage originating at such sides may occur in the semiconductor device. The formation of continuous grooves causes deterioration in strength and may lead to damage to the semiconductor substrate during handling.
Japanese Patent Laid-Open No. 2004-165227 discloses the formation of two grooves on separating lines on a substrate corresponding to each of the four sides of each semiconductor device. However, in this case, since the grooves are not continuous, the rectilinearity of separation decreases, resulting in nonuniform shapes of semiconductor devices after separation and, in particular, inhomogeneous dimensions of the respective sides. As a result, for example, the pick-up rate of collets picked up at the sides of semiconductor devices in a subsequent process declines, resulting in lower productivity.